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Respiration in Humans

Humans have a well developed respiratory system.

  Respiratory Organs of the Human Being

Organs of Human Respiratory System

The principal organs of respiration in mammals are the lungs. They are large organs occupying the major part of the chest or thoracic cavity. The thoracic cavity is bounded on the sides by the ribs and below by the muscular diaphragm. The lungs are spongy and consist of many branching air passages called bronchioles. The folded walls of the lung are made up of many air sacs or alveoli, and an extensive network of the blood vessels and the capillaries held together by connective tissue. Air enters the body through the nose and the pharynx and then goes into the windpipe or trachea. The opening into the windpipe is a narrow slit called the Glottis. Through the glottis, air passes into the trachea via the larynx or voice box. The pharynx is the common passage for both food and the air. A leaf like cartilaginous organ called epiglottis guards the opening of the larynx. During the course of swallowing food materials, the epiglottis keeps the larynx closed so that no food finds its way to the trachea. Any material entering the trachea causes 'choking' and 'coughing'. The trachea is a hollow tube extending throughout the length of the neck and a part of the thoracic cavity. Its wall is protected externally by a skeleton made of half rings of cartilage. The trachea after entering the thoracic cavity divides into two branches called bronchi. Each bronchus extends into each lung and subdivides into countless small bronchial tubes. The bronchial tubes again divide into many fine tubes called bronchioles. These bronchioles end in an air sac. The passage of air from outside to the lungs is as follows:

Nose----Pharynx----Trachea-----Bronchi------Bronchioles-----Alveolus of the lungs.

The Lungs
The lungs are a pair of conical hollow organs, the right one being larger than the left one. The lower surfaces of the lungs are concave to accommodate the diaphragm which divides the body cavity into the thoracic and abdominal cavities. The lungs are enclosed in a double layered membrane called pleura. The lungs are well protected in the body case consisting of the breast bone on the front (ventral side) and the ribs on the sides (lateral sides) and the vertebral column on the back (dorsal side).


As indicated earlier, major branches from the trachea enter the root of a lung and divide and subdivide to give progressively smaller bronchi and still smaller bronchioles. The small terminal bronchioles branch into respiratory bronchioles which branch again into alveolar ducts ending ultimately into alveoli.



Structure of Lungs and Alveoli
A. Gross Structure
B. Magnified View of a Part of the Lungs

The respiratory tract is lined internally with a mucous membrane. There are numerous ciliated cells lining the passage of the trachea and bronchi. The mucous membrane which is kept moist always traps any small particles of dust or microbes in the air so that these do not enter the lung alveoli. The ciliated cells in the trachea and bronchi push up these particles.

The epithelium in the conducting bronchioles (up to the terminal bronchioles) is pseudo-stratified and columnar. The epithelium decreases in height as the tubules become smaller in diameter. In the terminal bronchioles the epithelium is a single layer of columnar or cuboidal cells. These possess cilia. The mucous cells are few in number and muscles are absent in the terminal bronchioles.  Elastic and reticular fibres and muscles are present but cartilage is totally absent.

In the alveoli, the epithelium is extremely thin and made up of a simple squamous non-ciliated epithelium.

The cartilaginous rings in the walls of the trachea and bronchi make them elastic and prevent them from collapsing. The air is allowed to pass freely.

Fine Structure of Lung Alveoli

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